Scientist, -- How smart are they?

Over the last forty years we have listen to tens of thousands of scientist say the words, due to “human activities.” These are indeed very smart people in their particular area. For example in just one periodical over the last 4 months the following five articles where printed blaming human activities for what is wrong with the world.

Manuel Lerdau, A Positive Feedback with Negative Consequences, Science 13 April 2007 316: 212-213
“... As more and more regions become influenced by human activities, the number of communities that suffer in this feedback loop will increase.”

Big Fish, Little Fish, Shellfish, Science 30 March 2007 315: 1764
“The loss of large predators from ecosystems, often caused by human activities, can have effects that cascade through the rest of the food chain. Myers et al. (p. 1846 )”

Keith P. Shine and William T. Sturges, CO2 Is Not the Only Gas, Science 30 March 2007 315: 1804-1805
“...wetlands, soils, and the ocean; methane concentrations have increased dramatically in the 20th century as a result of human activities,...”

If all these scientists are right, why haven’t they considered the obvious, that the problem is not the activities, but the number of humans doing them.

The obvious problem here is that there are just 4 billion too many of them. If we had the technology of today with the population of 100 years ago, the world would be a far better place. It is impossible to reduce the carbon footprint sufficiently to save humankind without drastically reducing the number of feet.